The present invention relates generally to hydraulic valves for use in fluid pressure operated systems and more particularly to hydraulic valves for the relief of both abnormally high and abnormally low pressure conditions in such systems.
A common problem in hydraulic circuits has been the provision of high pressure protection devices for venting fluid to a reservoir where the circuit is subjected to a higher pressure level than desired. Many devices have been utilized in the prior art for high pressure protection. Generally, these devices can be classified as either (1) direct-acting, utilizing a poppet loaded directly against a spring, or (2) pilot-operated, utilizing a poppet loaded by a reference pressure.
In addition to high pressure protection, the need has also existed for protection against abnormally low pressure conditions which result in cavitation. This condition exists when fluid leaves one side of a hydraulic cylinder and is not replaced at an equal rate on the opposite side. A known solution to the cavitation problem is the use of a one-way check valve which permits fluid to enter the voided cylinder from a reservoir and simultaneously prevents fluid from leaving the cylinder under pressure.
Therefore, complete protection of a hydraulic cylinder requires both high pressure protection and cavitation prevention on both cylinder ends. Combination valve assemblies providing both high and low pressure protection within a single package are also well known in the art. However, to date such combination assemblies have not performed as well as individual valve units due to the following problems: (1) To date, such combination valve assemblies have had to rely in part on dynamic sealing. The resulting seal friction has interfered with the performance of these valves. (2) Undue passageway restrictions, and (3) other problems resulting from limitations on the size of the valve package have limited performance.